Line of sight radio communication over large or wide geographical coverage areas has posed significant problems in the past. Systems of this type generally must make communication possible between mobile or portable equipment which have low power transmitters and which can move in any direction within the wide coverage area. System configurations which have been used to make such communication possible include systems having a centralized fixed-site receiver and high power transmitter and a plurality of remotely located receivers connected by wireline or microwave back to the central site, systems including mobile repeaters which rebroadcast the radio communications, and systems which simultaneously transmit the radio communication or message from multiple fixed sites on the same frequency. Systems of this last type have often been referred to as "simulcast" systems.
Simulcast systems generally include a plurality of radio transmission and reception fixed sites located in spaced apart relation throughout the geographical area to be provided with radio communication coverage. Ideally, the fixed sites are distributed such that areas not covered by one site will be covered by another site. As a result, the sites are distributed to create an overlapping patchwork of individual radio coverage areas which, when taken together, approximates the desired geographic coverage area. The message to be transmitted from each site is conveyed from one site called the master site to each of the other sites by microwave or wireline and subsequently transmitted by all of the sites on the same frequency to be received by all of the mobiles and other portables within the wide coverage area.
Unfortunately, some problems are encountered with such simulcast systems. Because it is virtually impossible to configure the coverage area of each fixed site exactly, overlapping of the individual coverage areas must be allowed to assure complete or total wide area coverage. The coverage area overlapping of simulcast systems causes problems. Because all of the fixed site transmitters transmit on the same frequency, radio frequency carrier beat note interference and demodulated audio phase cancellation can result in the overlapping areas. Correction of these problems is expensive. For example, expensive frequency standards, linear phase microwave multiplex equipment or compensated telephone lines and phase delay equipment are required. The present invention avoids these problems by utilizing a different transmit channel frequency at each of the fixed sites and a trunked access system.
Trunked radio systems are well known. In such systems, a fixed number of duplex radio channels is shared among a large number of potential users. The system through a central controller allocates the radio channels to the users depending on their communication need and deallocates the radio channel upon the completion of a user's transmission. Communication between users and the central controller occurs on a designated link which can be a separate duplex control channel or some form of subaudible, audible, supra-audio, or subcarrier shared radio channel usage.
It is therefore a general object of the present invention to provide a new and improved wide coverage area radio communication system.
It is a further object of the present invention to provide such a system having a plurality of fixed transmission sites and wherein each fixed transmission site is arranged to transmit on a different respective frequency to avoid beat note interference and phase cancellation in overlapping coverage areas.
It is a further object of the invention to provide such a system wherein access of the system is allocated by a trunking system.
It is another object of the present invention to provide a new and improved method of relaying a message substantially simultaneously from a plurality of fixed transmitter sites over a wide geographical area.
It is another object of the invention to provide a method of relaying a message by radio communication from one mobile to at least another mobile over a wide geographical coverage area without requiring special audio conditioning or frequency control standards.